Rather than being stuck with the same character we're born with, our personalities change throughout our lives  in some cases more as we get older  a U.S. study has revealed.

A team of psychologists led by Dr Sanjay Srivastava from the University of California, Berkeley, surveyed a huge sample of 132,515 volunteers on the major groupings of personality traits known as the 'Big Five': conscientiousness, agreeableness, neuroticism, openness, and extroversion.

The biological view is the so-called 'plaster' hypothesis, where personality is genetically determined and 'sets' or matures after 30. In contrast, contextual theories argue that personality is determined by a number of influences, such as life stages, experiences, social environment, and gender.

The volunteers, residents of the U.S. and Canada aged between 21 and 60 years, were recruited through the internet. Although internet users are not perfectly representative of the general population, they are quite diverse and the study required a large sample which the internet could provide. 54% of the respondents were female, 86% Caucasian, and 14% Asian, African-American, Latino, or Middle Eastern.

Data was collected from websites where participants completed self-scoring questionnaires. The study was designed to test if the average levels of personality traits differ by age, and whether those age effects are different for men and women.

"Average levels of personality traits changed gradually but systematically throughout the lifespan, sometimes even more after age 30 than before," the researchers wrote.

"We believe adult personality is characterised more by plasticity than by increasing calcification, and the mechanisms of personality change can be understood best by considering the life contexts that accompany change," they concluded.

The results

Conscientiousness  being organised and disciplined, and linked to effective work performance  increased most strongly at younger than older ages although it improved throughout the entire age range. The researchers suggest the strong increase corresponds with the stage in life when people enter committed relationships and advance in the work force.

Agreeableness  warm, generous, helpful  accelerated during the twenties, peaked in the thirties, and slowed while continuing to increase in the forties. This coincides with the ages at which people are generally giving birth and nurturing children. Overall, women recorded higher levels of agreeableness than men, and both men and women were more agreeable later in life than when younger.

Neuroticism  a trait defined in people who worry and are emotionally unstable  was expected by the researchers to be the strongest candidate for interaction between gender and age. Indeed, women started out with higher neuroticism, which decreased with age, while men did not change substantially throughout life.

Openness  this declined with age for both men and women, but only slightly. Men began adulthood with a bit more openness than women, but then declined at a faster rate.

Extroversion  a personality trait marked by an outward-looking and gregarious nature  indicated that men became slightly more extroverted with age whereas women became slightly less extroverted, resulting in the gender differences lessening over time.